Resilient, acrylic graft polymers are produced by a multi-stage, sequential polymerization technique which comprises alternately producing resilient and nonresilient layers around an acrylic core material. Such materials are often referred to as “core/shell particles” or “core/shell tougheners”. These resilient polymers are ordinarily mixed with a hard nonresilient thermoplastic methacrylic matrix resin in order to provide toughness in articles molded from the resulting blend. When properly dispersed, the resilient acrylic graft polymer greatly improves the impact strength of the hard matrix resin while maintaining a balance of important physical properties of heat distortion temperature, flexural modulus, tensile strength and tensile elongation.
A number of patents and papers address polymer morphology that can be obtained by multi-stage emulsion polymerization. For a detailed discussion, see P. Lovell, M. El-Aasser Emulsion Polymerization and Emulsion Polymers, John Wiley and Sons, 1997, chapter 19. See also U.S. Pat. Nos. 5,625,001 and 5,998,554, 3,678,133, 3,793,402, 3,808,180, 3,985,703, 4,180,494, 4,543,383 and World Patent Application 99/12986. The above art provides a description of monomers, emulsifiers, graft-linking monomers, cross-linking monomers, initiators, and the like, useful in making staged (meth)acrylic emulsion polymers.
Polymeric MBLs are characterized by high glass transition temperatures, but are often quite brittle. See for instance U.S. Pat. No. 5,880,235 and the discussion at columns 1-3, and D. Arnoldi, et al., Kunststoffe, vol. 87, p. 734-736 (1997). Commercially available acrylic graft copolymers, such as the Paraloid® series of products sold by Rohm and Haas Co., Philadelphia, Pa., U.S.A., can be used to toughen MBL-containing polymers. However, an important criterion for toughening is the compatability of the shell of the core/shell particle with the matrix polymer. Poor compatibility leads to particle agglomeration, which results in poor physical properties in the resulting molded product. Thus, if one could use multistage graft copolymers derived from MBLs to toughen matrix resins containing MBLs without compromising their other superior properties, useful compositions would result. In addition, functional groups can be incorporated in the outer shell of the MBL-containing graft copolymer to improve compatibility with other thermoplastic polymers such as polyolefins, polyamides, polyesters, polycarbonates, polystyrenes, ABS-type polymers, polyacetals, polyethers, polyurethanes, poly(vinyl chloride), blends thereof and mixtures thereof.
Another aspect of the present invention is a process for preparing hard, nonresilient, cross-linked polymers from MBLs. These polymers have a fixed particle size and act as nano-sized (less than about 500 nanometer), pre-formed, high glass transition temperature (Tg) amorphous “filler” materials. When properly blended with thermoplastic resins, the resulting materials display much higher heat distortion temperatures than the thermoplastic alone.
This invention further discloses an emulsion polymerization process of preparing MBL-based homopolymers and copolymers. A general emulsion polymerization process is described in “Polymer Chemistry,” by Seymour and Carraher, fifth edition, Marcel Dekker, Inc. NY, 2000 and “Introduction to Polymers,” Young and Lovell, second edition, Chapman & Hall, 1991.
U.S. Pat. No. 3,444,148 to Adelman in Example 10 teaches synthesis of a ⅙(w/w) γ,γ-bis-trifluromethyl MBL/acrylonitrile copolymer via emulsion polymerization. However, there is no teaching for synthesis of higher MBL content copolymers and MBL homopolymers via emulsion polymerization. The distinctive feature of emulsion polymerization is the ease with which comonomer ratios can be adjusted during the reaction. For a detailed discussion, see C. B. Bucknall, Toughened Plastics, Applied Science Publishers, 1977, p. 99. In addition, the heats of reaction, particle size, molecular weight, and molecular weight distribution can be carefully controlled. Within the scope of the present invention, the MBL core/shell latex can be easily mixed with the MBL copolymer latex in the desired ratio and then coagulated together. Although this is commonly done, for example, in the manufacture of acrylonitrile/butadiene/styrene based polymers via emulsion, (see P. Lovell, M. El-Aasser Emulsion Polymerization and Emulsion Polymers, John Wiley and Sons, 1997, p. 668-670), the process has never been suggested for the preparation of MBL-based homopolymers and copolymers.
Among the uses of thermoplastics are those in which the optical properties of the polymer are important, particularly when the polymer is an optically clear material with little distortion of optical images. Such polymers, for example poly(methyl methacrylate) (PMMA) and certain polycarbonates are used as substitutes for glass where toughness is important, such as for safety glazing and signage. In uses where toughness is important, additional properties such as weather and/or heat resistance may also be important. For example, if a material needs to be thermally sterilized, it must withstand the temperature of the sterilization process. Polycarbonates often have poor weathering and/or hydrolysis resistance, while PMMA has a relatively low glass transition temperature (Tg), so its heat resistance is poor. Thus, polymers with a combination of good optical properties, and heat and weathering resistance are desired.
An important criterion for optical clarity is that the refractive index (RI) of the acrylic graft copolymer match that of the matrix thermoplastic to within 0.1% of refractive index units. This can be done by adjusting the ratio of styrene (RI of 1.590) to n-butyl acrylate (RI of 1.466) in the rubbery phase of the acrylic graft copolymer. While this technique is common to PMMA (RI 1.490), it has not yet been demonstrated with polymers of MBL (RI of 1.540) or polymers of γ-methyl MBL (RI of 1.510). The present invention teaches a process for making transparent polymer compositions can be obtained from multi-stage graft copolymer blends of MBL and γ-methyl-MBL (Me-MBL).
The acrylic copolymers and compositions of the invention are of use as molded parts, thermoform parts, sheets, films, foams, containers, bottles, pipes, profiles and other articles made in accordance with the invention.